A watermark in an audio or video signal can be detected at receiving or decoder-side using correlation, as described for example in WO 2007/031423, WO 97/33391 (U.S. Pat. No. 6,584,138 B1) or U.S. Pat. No. 6,061,793.
Many watermarking systems make use of correlation for calculating a detection metric, which means that several pseudo-random sequences or reference patterns are generated at encoder side and one or more of them are embedded inside the content (e.g. an audio or video signal), dependent on the message to be embedded. The same pseudo-random sequences are generated at decoder side. Frequency transform may be used to encode and decode the embedded message. To decode the embedded message, it is necessary to discover which pseudo-random sequence or sequences were embedded at encoder side. This is determined in these systems by correlating the known pseudo-random sequences with the possibly watermarked content, whereby the correlation may operate on a pre-processed version of the content, and that pre-processing may include inverse frequency transform, spectral shaping and/or whitening.
Each embedded reference pattern may represent a single bit of the embedded message. There are watermarking systems in which each embedded reference pattern is representing two or more bits of the embedded message.
In WO2005/078658, clusters of correlation results are evaluated, a cluster including correlation results in the vicinity of a correlation peak and exceeding a detection threshold.
A watermark detector decides, depending on the size of the correlation result values, whether or not a given pseudo-random sequence was embedded.
A related improved decision processing is described in PCT/US2007/014037, where the calculation of relative correlation result values decreases the false positive rate, i.e. the probability to classify a non-watermarked content as watermarked. It is believed that this processing works very well even if the watermarked content is altered for example by perceptual coding (like mp3, AAC, WMA, AC-3, MPEG).